This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
Transit time ultrasonic flow meters are capable of high accuracy performance over a wide range of application conditions. This has led to their adoption in applications such as custody transfer of hydrocarbons, and measurement of nuclear feedwater flows.
To achieve high accuracy it is common for transit time ultrasonic flow meters to employ multiple pairs of transducers to infer velocity on a number of discrete paths. The velocity measurements can then be combined, along with information on geometry, to produce a measure of volumetric flowrate.
Two features of ultrasonic meters are particularly attractive in many applications. Firstly, they can be designed to be non-intrusive, that is to present no blockage to the flow, and consequently produce insignificant pressure loss. Secondly, their self-diagnostic capabilities are attractive in applications where routine in-situ calibration is difficult for practical or cost reasons.
Currently the self-diagnostic capabilities of transit time ultrasonic meters are based on evaluation of parameters such as amplifier gain, signal-to-noise ratios, and velocity profile descriptors such as flatness, asymmetry and swirl [Peterson, S, Lightbody, C, Trail, J and Coughlan, L (2008) On-line condition based monitoring of gas USM's, Proceedings of the North Sea Flow Measurement Workshop, Scotland, October 2008; Kneisley, G, Lansing, J, Dietz, T (2009) Ultrasonic meter condition based monitoring—a fully automated solution, Proceedings of the North Sea Flow Measurement Workshop, Norway, October 2009.]. However, as these parameters are difficult to relate directly to the uncertainty of the flow measurement, the use of meter diagnostics alone is not presently regarded as sufficient as a means of flow meter verification. For example, in the UK the Measurement Guidelines of the offshore oil and gas measurement regulator, while recognizing the benefits of current diagnostic techniques, note that they have the disadvantage that “diagnostic facilities are presently qualitative, rather than quantitative” [Department of Trade and Industry, Licensing and Consents Unit, Guidance Notes for Petroleum Measurement Under the Petroleum (Production) Regulations, December 2003, Issue 7.]. In order to overcome this limitation, sometimes two flow meters are installed in series, i.e. with one a short distance downstream of the other. This allows the volumetric flowrates from the two flow meters to be compared with one another, with the result that the verification is quantitative, rather than qualitative. Taking this concept a step further, it has also been known to calculate two independent flow rate measurements using two independent subsets of transducers installed in a single meter body.
One example of such a meter design is the combination of a 4-path meter and a single path meter [Kneisley, G, Lansing, J, Dietz, T (2009) Ultrasonic meter condition based monitoring—a fully automated solution, Proceedings of the North Sea Flow Measurement Workshop, Norway, October 2009], as illustrated in FIG. 1. A disadvantage of this design is that the single path meter is much more sensitive to distortions of the flow velocity field than the 4-path meter. This difference in sensitivity means that when a difference is detected, there exists the possibility that the single path meter can be affected by a distortion of the flow field that has a negligible effect on the 4-path meter. In the case where the four path meter is used as the primary measurement, this could result in false alarms, i.e. the difference detected does not reflect a reduction in accuracy of the 4-path meter. For example, in the referenced paper it is shown that when a flow conditioner upstream of the meter has one hole become blocked, there is virtually no effect on the 4-path meter, whereas the effect on the single-path meter can be greater than 0.85%. If, for example, an alarm threshold of 0.5% was set for the difference between the 4-path and single path result, the outcome would be an alarm annunciation where in fact the 4-path meter is continuing to read accurately.
Other examples of this concept include using two similar but separate groups of ultrasonic paths, such as shown in FIGS. 2a, 2b, 3a and 3b. FIGS. 2a and 2b show an arrangement of eight paths where one set of four paths are all set at a first angle relative to the pipe axis and the second set of four paths are all set at the negative value of that angle, such that the paths form a symmetrical X about the pipe axis when viewed from above. In this example the first set of four paths would be 1, 2, 3 and 4 and the second set 5, 6, 7 and 8. In FIG. 3 an alternative arrangement is used whereby each independent set of four paths has paths selected alternately relative to the pipe axis. In FIGS. 3a and 3b, the first set of four paths would be A1, B1, C1 and D1 and the second set A2, B2, C2 and D2. However, both of the arrangements shown suffer from a common weakness in that each group of four paths will still be affected differently by distortions of the flow velocity field, particularly when complex non-axial flow fields such as asymmetric rotational are present. What will happen in such a case is that one group of four paths will produce a result that will overestimate the flow rate, whilst the other group will underestimate the flow rate. Whilst this has some use in diagnosing flow conditions, it complicates the process of meter verification, as it is difficult to distinguish between an error in the measurement system itself and a difference that is created by the flow velocity field.
This limitation can be reduced in magnitude by use of a mechanical flow conditioning element installed upstream of the flow meter is employed to reduce the transverse flow components, but this negates the benefits of a non-intrusive meter design.
A further disadvantage of the concept using two similar groups of ultrasonic paths such as shown in FIGS. 2a, 2b, 3a and 3b is that even when a flow conditioner is used, some problems may be difficult to detect or quantify. If, for example, a uniform buildup of contamination inside the meter body was to occur, then the output from each set of four paths would be affected equally, and detection of the problem would have to rely on qualitative diagnostics such as amplifier gain, velocity profile shape or comparison of sound velocities, as there would be no indicated difference in flow rates.